David McKay

[Ir(PCy3)2(H)2(H2BNMe2)]+ as a Latent Source of Aminoborane: Probing the Role of Metal in the Dehydrocoupling of H3B⋅NMe2H and Retrodimerisation of [H2BNMe2]2

The Ir(III) fragment {Ir(PCy(3))(2)(H)(2)}(+) has been used to probe the role of the metal centre in the catalytic dehydrocoupling of H(3)B⋅NMe(2)H (A) to ultimately give dimeric aminoborane [H(2)BNMe(2)](2) (D). Addition of A to [Ir(PCy(3))(2)(H)(2)(H(2))(2)][BAr(F)(4)] (1; Ar(F) = (C(6)H(3)(CF(3))(2)), gives the amine-borane complex [Ir(PCy(3))(2)(H)(2)(H(3)B⋅NMe(2)H)][BAr(F)(4)] (2 a), which slowly dehydrogenates to afford the aminoborane complex [Ir(PCy(3))(2)(H)(2)(H(2)B-NMe(2))][BAr(F)(4)] (3). DFT calculations have been used to probe the mechanism of dehydrogenation and show a pathway featuring sequential BH activation/H(2) loss/NH activation. Addition of D to 1 results in retrodimerisation of D to afford 3. DFT calculations indicate that this involves metal trapping of the monomer-dimer equilibrium, 2 H(2)BNMe(2) ⇌ [H(2)BNMe(2)](2). Ruthenium and rhodium analogues also promote this reaction. Addition of MeCN to 3 affords [Ir(PCy(3))(2)(H)(2)(NCMe)(2)][BAr(F)(4)] (6) liberating H(2)B-NMe(2) (B), which then dimerises to give D. This is shown to be a second-order process. It also allows on- and off-metal coupling processes to be probed. Addition of MeCN to 3 followed by A gives D with no amine-borane intermediates observed. Addition of A to 3 results in the formation of significant amounts of oligomeric H(3)B⋅NMe(2)BH(2)⋅NMe(2)H (C), which ultimately was converted to D. These results indicate that the metal is involved in both the dehydrogenation of A, to give B, and the oligomerisation reaction to afford C. A mechanism is suggested for this latter process. The reactivity of oligomer C with the Ir complexes is also reported. Addition of excess C to 1 promotes its transformation into D, with 3 observed as the final organometallic product, suggesting a B-N bond cleavage mechanism. Complex 6 does not react with C, but in combination with B oligomer C is consumed to eventually give D, suggesting an additional role for free aminoborane in the formation of D from C.

Room-temperature C-C bond cleavage of an arene by a metallacarborane.

The activation and cleavage of C C bonds by transition-metal species is an area of intense current interest, and, although they are still relatively rare, a number of systems that afford the breaking of C C single bonds are known. In contrast, the cleavage of aromatic C C bonds is considered to be extraordinarily difficult. Six-carbon aromatic rings can be cleaved in the gas phase at high temperatures, whereas under less extreme conditions such rings are cleaved by enzymes, an important part of the global carbon cycle. However, there are very few reports of low-temperature cleavage reactions of aromatic rings in nonbiological systems. Of the examples that are known, (biomimetic) oxidative cleavage is the more common process but is generally regarded as difficult to control, whereas reductive cleavage is much more rare. However, both oxidative and reductive cleavage reactions typically involve significant initial chemical modification of the aromatic ring. Sattler and Parkin recently described the cleavage of a C C bond in an aromatic heterocycle (a quinoxaline) at 90 8C. We report herein the unprecedented cleavage of an aromatic C C bond in a simple arene at room temperature by a metallacarborane without other chemical modification to the arene. Treatment of 1,1’-bis(o-carborane), 1-(1’,2’-closoC2B10H11)-2-closo-C2B10H11 (Figure 1), with an excess of Li in THF in the presence of naphthalene and subsequent reaction with [{Ru(p-cymene)Cl2}2] (p-cymene = 1-iPr,4MeC6H4), affords the dark red metallacarborane 1-(1’,2’closo-C2B10H11)-4-{C10H14Ru(p-cymene)}-4,1,6-closoRuC2B10H11 as the only isolable product (in ca. 20% yield) after workup (involves TLC methods). The product was characterized by mass spectroscopy, H and B{H} NMR spectroscopy, and ultimately by single-crystal X-ray diffraction. In the H spectrum there are, in addition to broad CcageH resonances at approximately d = 4.7 and 2.2 ppm, the normal resonances assigned to the CH3C6H4CHMe2 protons of a h -pcymene ligand in an asymmetric complex (four dd between d = 6.5 and 5.5 ppm with J and J couplings of ca. 6 Hz and 1.5 Hz, respectively). However, the signals normally assigned to the CH3C6H4CHMe2, CH3C6H4CHMe2, and CH3C6H4CHMe2 protons all appear doubled. In addition there are two high-frequency doublet resonances (d = 9.6, 9.4 ppm) and two additional resonances (d = 4.5, 4.1 ppm) which appear as apparent triplets. Collectively these data suggest that there are two different C10H14 units in the product; one is a regular h-p-cymene ligand but the other appears to have been subjected to a major structural change. The B{H} spectrum is relatively uninformative with nine resonances between d = 6 and 25 ppm, including a multiple signal at d = 10.6 ppm that accounts for ten boron atoms. The mass spectrum confirms the molecular formula as C25H50B20Ru2 (envelope centered on m/z 757) which implies bis(carborane) plus two {RuC10H14} units, but, as is evident from the NMR spectra, the molecule is asymmetric and one of the p-cymene ligands has been substantially altered. A crystallographic study resulted in the molecular structure shown in Figure 2. Figure 3 shows an alternative view of the central part of the molecule. The molecule consists of a 13-vertex docosahedral ruthenacarborane (cage A) with a 4,1,6-RuC2B10 architecture, the

Interplay between Cationic and Neutral Species in the Rhodium‐Catalyzed Hydroaminomethylation Reaction

The reactivity of [Rh(CO)(2){(R,R)-Ph-BPE}]BF(4) (2) toward amine, CO and/or H(2) was examined by high-pressure NMR and IR spectroscopy. The two cationic pentacoordinated species [Rh(CO)(3) {(R,R)-Ph-BPE}]BF(4) (4) and [Rh(CO)(2)(NHC(5)H(10)){(R,R)-Ph-BPE}]BF(4) (8) were identified. The transformation of 2 into the neutral complex [RhH(CO)(2){(R,R)-Ph-BPE}] (3) under hydroaminomethylation conditions (CO/H(2), amine) was investigated. The full mechanisms related to the formation of 3, 4 and 8 starting from 2 are supported by DFT calculations. In particular, the pathway from 2 to 3 revealed the deprotonation by the amine of the dihydride species [Rh(H)(2)(CO)(2){(R,R)-Ph-BPE}]BF(4) (6), resulting from the oxidative addition of H(2) on 2.

Computational study of the hydrodefluorination of fluoroarenes at [Ru(NHC)(PR3)2(CO)(H)2]: predicted scope and regioselectivities

Density functional theory calculations have been employed to investigate the scope and selectivity of the hydrodefluorination (HDF) of fluoroarenes, C6F(6-n)H(n) (n = 0-5), at catalysts of the type [Ru(NHC)(PR3)2(CO)(H)2]. Based on our previous study (Angew. Chem., Int. Ed., 2011, 50, 2783) two mechanisms featuring the nucleophilic attack of a hydride ligand at a fluoroarene substrate were considered: (i) a concerted process with Ru-H/C-F exchange occurring in one step; and (ii) a stepwise pathway in which the rate-determining transition state involves formation of HF and a Ru-σ-fluoroaryl complex. The nature of the metal coordination environment and, in particular, the NHC ligand was found to play an important role in both promoting the HDF reaction and determining the regioselectivity of this process. Thus for the reaction of C6F5H, the full experimental system (NHC = IMes, R = Ph) promotes HDF through (i) more facile initial PR3/fluoroarene substitution and (ii) the ability of the NHC N-aryl substituents to stabilise the key C-F bond breaking transition state through F···HC interactions. This latter effect is maximised along the lower energy stepwise pathway when an ortho-H substituent is present and this accounts for the ortho-selectivity seen in the reaction of C6F5H to give 1,2,3,4-C6F4H2. Computed C-F bond dissociation energies (BDEs) for C6F(6-n)H(n) substrates show a general increase with larger n and are most sensitive to the number of ortho-F substituents present. However, HDF is always computed to remain significantly exothermic when a silane such as Me3SiH is included as terminal reductant. Computed barriers to HDF also generally increase with greater n, and for the concerted pathway a good correlation between C-F BDE and barrier height is seen. The two mechanisms were found to have complementary regioselectivities. For the concerted pathway the reaction is directed to sites with two ortho-F substituents, as these have the weakest C-F bonds. In contrast, reaction along the stepwise pathway is directed to sites with only one ortho-F substituent, due to difficulties in accommodating ortho-F substituents in the C-F bond cleavage transition state. Calculations predict that 1,2,3,5-C6F4H2 and 1,2,3,4-C6F4H2 are viable candidates for HDF at [Ru(IMes)(PPh3)2(CO)(H)2] and that this would proceed selectively to give 1,2,4-C6F3H3 and 1,2,3-C6F3H3, respectively.

Unprecedented steric deformation of ortho-carborane

The reduction and subsequent oxidation of meta-carboranes containing bulky groups attached to the cage C atoms affords sterically-crowded ortho-carboranes with unprecedentedly long C-C connectivities.

Supraicosahedral indenyl cobaltacarboranes

13-vertex indenyl cobaltacarboranes with 4,1,6-, 4,1,10- and 4,1,2-CoC(2)B(10) architectures have been synthesised by reduction of the corresponding closo carborane and metallation with an {(eta-C(9)H(7))Co} fragment. Variants of the 4,1,6-isomer were prepared with no, one and two methyl groups on cage C atoms, whilst 4,1,2-species were obtained both with two methyl groups and a trimethylene tether on the cage C atoms. Thermolysis of the 4,1,6-isomers yielded the corresponding 4,1,8-isomers, which in turn were converted to 4,1,12-isomers by thermolysis at higher temperatures. Alternatively relatively mild heating of the 4,1,10-isomer led to the 4,1,12-isomer directly. Products were characterised by mass spectrometry, (1)H and (11)B NMR spectroscopies and, in most cases, elemental analysis, and nine compounds were studied crystallographically. The 4,1,6-, 4,1,8-, 4,1,10- and 4,1,12- species have docosahedral cages whilst the 4,1,2-species are henicosahedral. In the structural studies attention focused on the orientation of the indenyl ligand with respect to the carborane ligand since this affords experimental information on the metal-cage bonding through the structural indenyl effect. There is a general tendency for the indenyl ligand to adopt orientations in which the ring junction C atoms lie trans to cage B atoms. In cases where the orientation is not compromised by the presence of a non-H substituent on the face of the carborane there is generally good agreement between the experimental orientation and that computed by DFT calculations for the related naphthalene ferracarboranes (eta-C(10)H(8))FeC(2)B(10)H(12). The presence of C-methyl substituents in the indenyl cobaltacarboranes tends to override this preference except in the case of 1,6-Me(2)-4-(eta-C(9)H(7))-4,1,6-closo-CoC(2)B(10)H(10) where the indenyl ligand instead is forced to incline away from the cage methyl groups. In DCM solution the 4,1,6-, 4,1,8-, 4,1,10- and 4,1,12- isomers of (eta-C(9)H(7))CoC(2)B(10)H(12) exhibit two, stepwise, 1-electron reductions assigned to Co(III)/Co(II)/Co(I) couples at less negative potentials than those of the corresponding Cp compounds. Moreover these reductions are easier for those isomers (4,1,6- and 4,1,10-) in which there are two cage C atoms in the carborane face to which the metal atom is bound. By spectroelectrochemical and EPR measurements it is concluded that the reductions of these indenyl cobaltacarboranes are largely metal-based.

New chemistry of 1,2-closo-P2B10H10 and 1,2-closo-As2B10H10; in silico and gas electron diffraction structures, and new metalladiphospha- and metalladiarsaboranes

The molecular structures of 1,2-closo-P(2)B(10)H(10) (1) and 1,2-closo-As(2)B(10)H(10) (2) have been determined by gas electron diffraction and the results obtained compared with those from computation at the MP2/6-31G** level of theory. The level of agreement is good for 2 (root-mean-square [rms] misfit for As and B atoms 0.0297 Å) and very good for 1 (rms misfit for P and B atoms 0.0082 Å). In comparing the structures of 1 and 2 with that of 1,2-closo-C(2)B(10)H(12) (I) it is evident that expansion of the polyhedron from I to 1 to 2 is restricted only to the heteroatom vertices and the B(6) face to which these are bound. Following deboronation (at B3) and subsequent metallation, compounds 1 and 2 have been converted into the new metalladiheteroboranes 3-(η-C(9)H(7))-3,1,2-closo-CoAs(2)B(9)H(9) (4), 3-(η-C(10)H(14))-3,1,2-closo-RuAs(2)B(9)H(9) (5), 3-(η-C(5)H(5))-3,1,2-closo-CoP(2)B(9)H(9) (6), 3-(η-C(9)H(7))-3,1,2-closo-CoP(2)B(9)H(9) (7) and 3-(η-C(10)H(14))-3,1,2-closo-RuP(2)B(9)H(9) (8), the last three constituting the first examples of metalladiphosphaboranes. Together with the known compound 3-(η-C(5)H(5))-3,1,2-closo-CoAs(2)B(9)H(9) (3), compounds 4-8 have been analysed by NMR spectroscopy and (except for 8) single-crystal X-ray diffraction. The (11)B NMR spectra of analogous pairs of metalladiphosphaborane and metalladiarsaborane (6 and 3, 7 and 4, 8 and 5) reveal a consistently narrower (9-10 ppm) chemical shift range for the metalladiarsaboranes, the combined result of a deshielding of the lowest frequency resonance (B6) and an increased shielding of the highest frequency resonance (B8) via an antipodal effect. In crystallographic studies, compounds 3 and 5B (one of two crystallographically-independent molecules) suffer As/B disorder, but in both cases it was possible to refine distinct, ordered, components of the disorder, the first time this has been reported for metalladiarsaboranes. Moreover, whilst the Cp compounds 6 and 3 are disordered, their indenyl analogues 7 and 4 are either ordered or significantly less disordered, a consequence of both the reduced symmetry of an indenyl ligand compared to a Cp ligand and the preference of the former for a distinct conformation relative to the cage heteroatoms. Unexpectedly, whilst this conformation in the cobaltadiphosphaborane 7 is cis-staggered (similar to that previously established for the analogous cobaltadicarborane), in the cobaltadiarsaborane 4 the conformation is close to cis-eclipsed.

Modelling and Rationalizing Organometallic Chemistry with Computation: Where Are We?

In this chapter, a perspective on how the field of applied computational organometallic chemistry has developed since the mid-1980s is presented. We describe the way in which the modelling of chemical systems has evolved over time, using metallocene chemistry as an example, and highlight the successes and limitations of simple models that were mandatory in the early days of the discipline. A number of more recent case studies are then presented where the full experimental system is now employed and a more quantitative outcome is sought. This includes examples from the Ce-mediated hydrogenation of pyridine, Rh-catalysed C–H bond activation and functionalization, Pd-catalysed azidocarbonylation and phenyl iodide activation at Ru(II) complexes. We conclude with our take on the title question.

Exceedingly Facile Ph-X Activation (X = Cl, Br, I) with Ruthenium(II): Arresting Kinetics, Autocatalysis, and Mechanisms.

Abstract [(Ph3P)3Ru(L)(H)2] (where L=H2 (1) in the presence of styrene, Ph3P (3), and N2 (4)) cleave the Ph—X bond (X=Cl, Br, I) at RT to give [(Ph3P)3RuH(X)] (2) and PhH. A combined experimental and DFT study points to [(Ph3P)3Ru(H)2] as the reactive species generated upon spontaneous loss of L from 3 and 4. The reaction of 3 with excess PhI displays striking kinetics which initially appears zeroth order in Ru. However mechanistic studies reveal that this is due to autocatalysis comprising two factors: 1) complex 2, originating from the initial PhI activation with 3, is roughly as reactive toward PhI as 3 itself; and 2) the Ph—I bond cleavage with the just‐produced 2 gives rise to [(Ph3P)2RuI2], which quickly comproportionates with the still‐present 3 to recover 2. Both the initial and onward activation reactions involve PPh3 dissociation, PhI coordination to Ru through I, rearrangement to a η2‐PhI intermediate, and Ph—I oxidative addition.

Room Temperature Regioselective Catalytic Hydrodefluorination of Fluoroarenes with trans-[Ru(NHC)4H2] through a Concerted Nucleophilic Ru-H Attack Pathway

The efficient and highly selective room temperature hydrodefluorination (HDF) of fluoroarenes by the trans-[Ru(IMe4 )4 H2 ] catalyst, 3, is reported. Mechanistic studies show 3 acts directly in catalysis without any ligand dissociation and DFT calculations indicate a concerted nucleophilic attack mechanism. The calculations fully account for the observed selectivities which corroborate earlier predictions regarding the selectivity of HDF.